kernel: Compare pointers with NULL in while statements

Make while statement using pointers explicitly check whether
the value is NULL or not.

The C standard does not say that the null pointer is the same
as the pointer to memory address 0 and because of this is a good
practice always compare with the macro NULL.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

kernel/pipes.c

@@ -477,7 +477,7 @@ int _k_pipe_put_internal(struct k_pipe *pipe, struct k_pipe_async *async_desc,
 
 	struct k_thread *thread = (struct k_thread *)
 				  sys_dlist_get(&xfer_list);
-	while (thread) {
+	while (thread != NULL) {
 		desc = (struct k_pipe_desc *)thread->base.swap_data;
 		bytes_copied = pipe_xfer(desc->buffer, desc->bytes_to_xfer,
 					  data + num_bytes_written,
@@ -625,7 +625,7 @@ int _impl_k_pipe_get(struct k_pipe *pipe, void *data, size_t bytes_to_read,
 
 	struct k_thread *thread = (struct k_thread *)
 				  sys_dlist_get(&xfer_list);
-	while (thread && (num_bytes_read < bytes_to_read)) {
+	while ((thread != NULL) && (num_bytes_read < bytes_to_read)) {
 		desc = (struct k_pipe_desc *)thread->base.swap_data;
 		bytes_copied = pipe_xfer(data + num_bytes_read,
 					  bytes_to_read - num_bytes_read,
@@ -665,7 +665,7 @@ int _impl_k_pipe_get(struct k_pipe *pipe, void *data, size_t bytes_to_read,
 	 * into the pipe's circular buffer.
 	 */
 
-	while (thread) {
+	while (thread != NULL) {
 		desc = (struct k_pipe_desc *)thread->base.swap_data;
 		bytes_copied = pipe_buffer_put(pipe, desc->buffer,
 						desc->bytes_to_xfer);

kernel/queue.c

@@ -237,7 +237,8 @@ void k_queue_append_list(struct k_queue *queue, void *head, void *tail)
 #if !defined(CONFIG_POLL)
 	struct k_thread *thread;
 
-	while (head && ((thread = _unpend_first_thread(&queue->wait_q)))) {
+	while ((head != NULL) &&
+		(thread = _unpend_first_thread(&queue->wait_q))) {
 		prepare_thread_to_run(thread, head);
 		head = *(void **)head;
 	}

kernel/sys_clock.c

@@ -196,7 +196,7 @@ static inline void handle_timeouts(s32_t ticks)
 	 * prohibited.
 	 */
 
-	while (next) {
+	while (next != NULL) {
 
 		/*
 		 * In the case where ticks number is greater than the first

kernel/thread.c

@@ -162,8 +162,8 @@ void _thread_monitor_exit(struct k_thread *thread)
 		struct k_thread *prev_thread;
 
 		prev_thread = _kernel.threads;
-		while (prev_thread != NULL &&
+		while ((prev_thread != NULL) &&
-		       thread != prev_thread->next_thread) {
+			(thread != prev_thread->next_thread)) {
 			prev_thread = prev_thread->next_thread;
 		}
 		if (prev_thread != NULL) {